Chemical Engineering February 2014 - 51

Table 2. sTraighT pipe lengTh requiremenTs for
flow disturbance
Thermal-dispersion mass flowmeTers in mulTiples of pipe diameTera
Thermal dispersion mass flowmeters
in-lineb
Single elbow
4:1 Reduction
4:1 Expansion
Control valved or P regulator
Two elbows in the same plane
Two elbows in different planese
1
3
3
3
3
5
15
15
30
40
20
40
insertion
upstream downstream upstream downstream upstream downstream
5
5
10
5
5
10
28
14
30
32
36
62
7
7
7
7
7
7
Notes to Table 1: (a) Requirements for the length of intervening straight pipe in multiples of pipe diameter at 1 bara pressure; consult manufacturer
for pressure effects; specifications may vary from manufacturer to manufacturer; (b) For an in-line meter with a built-in flow conditioner consisting of
two upstream separated perforated plates; (c) For comparison purposes only; based on ISO standard 5167 [10]; (d) If the control valve is always wide
open, base the length requirement on the valve's inlet or outlet fitting size; (e) For three elbows, the required length is doubled.
valve facilitating installation, insertion,
retraction and removal of
the insertion flowmeter from an active
process pipe line without interruption
of the flow or the leakage of
process gas. The retraction mechanism
provides operator safety for
pressurized process lines.
Flow calibration
Because the critical dimensions of
the flow sensor of TD mass flowmeters
are so small, manufacturing
technology is generally incapable
of maintaining sufficiently small
tolerances to ensure a high degree
of reproducibility from flow sensor
to flow sensor. Additionally, the internal
diameters of the pipes used
in in-line flow bodies have substantial
variations. For these reasons,
every general-purpose TD mass
flowmeter is flow calibrated by the
manufacturer, just like most other
kinds of flowmeters. Exceptions
may include flow switches and lowaccuracy
flowmeters.
The gas-flow calibration facilities
of manufacturers, flow calibraReferences
1.
King, L. V., On the convection of heat from
small cylinders in a stream of fluid: Determination
of the convection constants of small
platinum wires with application to hot-wire
anemometry, Phil. Trans. Roy. Soc., A214, pp.
373-432, 1914.
2. Olin, J. G., Thermal anemometry, in " The
Measurements, Instrumentation, and Sensors
Handbook, " ed. Webster, J. G., pp. 29-18
to 29-37, CRC Press, Boca Raton, 1999.
3. ASME MFC-21.2-2010. Measurement of
fluid flow by means of thermal dispersion
mass flowmeters.
4. ASME MFC-21.1M. Measurement of fluid
flow by means of capillary tube thermal
mass flowmeters and controllers.
5. ASME MFC-15M. Insertion metering.
6. Olin, J. G., A thermal mass flow monitor for
continuous emissions monitoring systems
(CEMS), Proc. ISA/93 International Conference,
Exhibition and Training Program, 93404,
pp.1,635-1,653, 1993.
7. Bruun, H. H., " Hot-Wire Anemometry: Principles
and Signal Analysis, " Oxford Univ.
Press, Oxford, U.K., 1995.
8. Olin, J. G., Industrial thermal mass flowmeters
- Part 1: Principles of operation, Measurements
and Control, 193, pp. 83-90, Feb.
1999.
9. Olin, J. G., Industrial thermal mass flowmeters
- Part 2: Applications, Measurements
and Control, 194, pp. 1-6, April, 1999.
10. Miller, R.W., " Flow Measurement Engineering
Handbook, " 3rd ed., McGraw Hill, N.Y.,
1996.
tion laboratories and users should
be capable of: (1) generating a stable,
steady-state, reproducible gas
mass flowrate; (2) accommodating
the entire mass flowrate range
specified; (3) having a flow calibration
standard that has an accuracy
at least three times better than
the flowmeter under test; and (4)
reproducing the gas composition,
temperature and pressure to be
encountered in the actual application.
Gas-flow calibration facilities
are of two types: open loop and
closed loop.
Closed-loop facilities are recommended
because they allow flow
calibration at elevated pressures
and temperatures, and with gases
other than air. The preferred pressurized
closed-loop system for
high-accuracy applications has
the following major components
(Figure 8), listed in flow sequence:
(1) a gas charging source, such as
a compressed gas tank; (2) a flow
source, such as a high-pressure
axial or centrifugal in-line pump;
(3) a flow controller, such as a precision
flow control valve or variable
speed motor drive; (4) flowconditioning
section(s) upstream
of the flow calibration standard(s);
(5) the in-line flow calibration
standard (more than one may be
needed to cover the mass flowrate
range); (6) a heating section, such
as an in-line electric heater; (7) a
flow-conditioning section upstream
of the flowmeter under test; and (8)
the flowmeter under test. Additionally,
the facility should have: accurate
gas temperature and pressure
instrumentation at both the flow
calibration standard and the flowmeter
under test; pressure relief
and venting components; a vacuum
pump for evacuating the system
prior to charging; and, optionally,
a cooling section downstream of
the flowmeter under test. Highaccuracy
in-line flow calibration
standards include custody-transfer
grade multi-path ultrasonic flowmeters;
turbine flowmeters; flow
nozzles; and positive-displacement
flowmeters.
■
Edited by Gerald Ondrey
Author
John G. Olin is the founder
and CEO of Sierra Instruments,
Inc. (5 Harris Court,
Building L, Monterey, California
93940; Phone: 831373-0200;
Fax: 831-373-4402;
Email: j_olin@sierrainstruments.com).
Olin received his
B.S degree from Illinois Institute
of Technology and his
M.S. and Ph.D. from Stanford
University, all in mechanical
engineering. At Stanford, Olin specialized
in fluid mechanics and heat transfer and used
hot-wire anemometers in research pursuant to
his doctoral dissertation. He founded Sierra Instruments
in 1973 with the purpose of offering
industrial-grade thermal dispersion mass flowmeters
to solve industry's need for rugged, reliable
flowmeters based on the thermal principle.
Olin has a dozen patents and over 60 publications
in the field.
ChemiCal engineering www.Che.Com february 2014 51
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